The use of a gas-cluster ion beam (GCIB) for etching, cleaning, and smoothing surfaces is known (see for example, U.S. Pat. No. 5,814,194, Deguchi, et al.) in the art. GCIBs have also been employed for assisting the deposition of films from vaporized carbonaceous materials (see for example, U.S. Pat. No. 6,416,820, Yamada, et al.) As the term is used herein, gas-clusters are nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such clusters may be comprised of aggregates of from a few to several thousand molecules or more, loosely bound to form the clusters. The clusters can be ionized by electron bombardment or other means, permitting them to be formed into directed beams of controllable energy. Such ions each typically carry positive charges of q·e (where e is the magnitude of the electronic charge and q is an integer of from one to several representing the charge state of the cluster ion). The larger sized clusters are often the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per molecule. The clusters disintegrate on impact, with each individual molecule carrying only a small fraction of the total cluster energy. Consequently, the impact effects of large clusters are substantial, but are limited to a very shallow surface region. This makes ion clusters effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage characteristic of conventional ion beam processing.
Means for creation of and acceleration of such GCIBs are described in the reference (U.S. Pat. No. 5,814,194) previously cited, the teachings of which are incorporated herein by reference. Presently available ion cluster sources produce clusters ions having a wide distribution of sizes, N, up to N of several thousand (where N=the number of molecules in each cluster—in the case of monatomic gases like argon, an atom of the monatomic gas will be referred to as either an atom or a molecule and an ionized atom of such a monatomic gas will be referred to as either an ionized atom, or a molecular ion, or simply a monomer ion—throughout this discussion).
Many useful surface-processing effects can be achieved by bombarding surfaces with GCIBs. These processing effects include, but are not necessarily limited to, smoothing, etching, film growth, and infusion of materials into surfaces. In many cases, it is found that in order to achieve industrially practical throughputs in such processes, GCIB currents on the order of hundreds or perhaps thousands of microamps are required. Experimental GCIB beam currents have been reported on the order of several hundreds or a few thousands of microamperes in the form of short duration transient beam bursts. But, for industrial productivity and high quality surface processing results, GCIB processing equipment for etching, smoothing, cleaning, infusing, or film formation must produce steady, long-term-stable beams so that GCIB processing of a workpiece surface can proceed for minutes or hours without interruption or beam current transients. GCIB processing equipment possessing such long-term stability has been heretofore limited to beam currents on the order of a few hundreds of microamperes. Attempts to form higher beam currents have heretofore generally resulted in beams without long-term stability and having frequent beam transients (commonly called “glitches”) resulting from arcing or other transient effects in the beamlines. Such transients can arise in a variety of ways, but their effect is to produce non-uniform processing of the workpieces or, in the case of severe arcing, even physical damage to or transient misbehavior of control systems in the GCIB processing systems.
Thus, there exists a need to provide methods and apparatus for improving the beam stability in high current GCIB workpiece processing systems. It is an object of the invention to fulfill such need.